Rob dropped sphere (3)

I believe the answer is:

orb

I'm a little stuck... Click here to teach me more about this clue!

'sphere' is the definition.
(orb is a kind of sphere)

'rob dropped' is the wordplay.
'rob' can be anagrammed to 'ORB'.
Though, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for orb that I've seen before include "Part of royal regalia" , "Sovereign artefact" , "Sphere - eye" , "Jewelled ornament" , "Sphere (found in regalia)" .)

I've seen this clue in The Mirror.

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